The present invention relates to printer devices, and particularly, although not exclusively, to a method and apparatus for detecting faulty nozzles in ink jet devices.
It is known to produce paper copies, also known as xe2x80x9chardxe2x80x9d copies, of files stored on a host device, eg a computer using a printer device. The print media onto which files may be printed includes paper and clear acetates for use in lectures, seminars and the like.
Referring to FIG. 1 herein, there is illustrated a conventional host device 100, in this case a personal computer, linked to a printer device 120 via a cable 110. Amongst the known methods for printing text or graphics and the like onto a print media such as paper it is known to build up an image on the paper by spraying drops of ink from a plurality of nozzles.
Referring to FIG. 2 herein, there is illustrated schematically part of a prior art printer device comprising an array of printer nozzles 220 arranged into parallel rows. The unit comprising the arrangement of printer nozzles is known herein as a print head 210. In a conventional printer of the type described herein, the print head 210 is constrained to move in a direction 260 with respect to the print media 200 eg a sheet of A4 paper. In addition, the print media 200 is also constrained to move in a further direction 250. Preferably, direction 260 is orthogonal to direction 250.
During a normal print operation, print head 210 is moved into a first position with respect to the print media 200 and a plurality of ink drops 230, 240 are sprayed from a number of printer nozzles 220 contained within print head 210. This process is also known as a print operation. After the completion of a print operation the print head 210 is moved in a direction 260 to a second position and another print operation is performed. In a like manner, the print head is repeatedly moved in a direction 260 across the print media 200 and a print operation performed after each such movement of the print head 210. In practice, modern printers of this type are arranged to carry out such print operations while the print head is in motion, thus obviating the need to move the print head discrete distances between print operations. When the print head 210 reaches an edge of the print media 200, the print media is moved a short distance in a direction 250, parallel to a main length of the print media 200, and further print operations are performed. By repetition of this process, a complete printed page may be produced in an incremental manner.
In order to maintain the quality of the printed output of the printer device, it is important that each instruction to the print head to produce an ink drop from a given nozzle does indeed produce such an ink drop. It is also important that each drop that is ejected from the print head is correctly positioned on the print media.
In conventional printers it is known to attempt to detect an ink drop as it leaves a nozzle of the print head during nozzle testing routines. In this manner, if no ink drop is detected in response to a signal to eject an ink drop, the nozzle concerned may be assumed to be malfunctioning and appropriate maintenance routines may be implemented. An example of this type of drop detection system is disclosed in European Patent Application No.1027987, in the name of Hewlett-Packard Company.
In such systems, the drop detection unit employs an LED and lens to produce a collimated beam of light. The collimated beam of light is arranged to be incident on a photo diode, which generates an electrical current in response to the incident light. Prior to testing nozzles of a print head, the print head is positioned in a testing position, generally outside of the region used for printing onto the print media. An ink drop is then sprayed from a selected nozzle of the print head through the collimated beam of light. As the ink drop passes through the light beam, it partially blocks light normally incident on the photo diode. Due to the decrease in light incident on the photo diode, the current which it generates decreases temporarily. The change in the output current of photo diode is detected and forms the basis for an ink drop detection signal which is generated and processed by a drop detection processor. This process is then repeated with each nozzle of the print head until each has been tested.
Thus, the above described type of drop detection devices may be used to determine whether particular nozzles are ejecting ink drops in response to firing signals. However, such devises do not generally distinguish between an ink drop that is ejected in the correct direction and an ink drop which is ejected in an incorrect direction, as might arise in the event that a nozzle is partly blocked by dried ink, or has been damaged in some way, for example by a print head crash.
As the skilled reader will understand, it is desirable to be able to correctly distinguish between nozzles that eject ink drops in correct and incorrect directions. In the first case, the drops will be correctly placed on the print media, whereas in the second case, the drops will not be correctly positioned on the print media, thus causing a degradation in the quality of the printed output. Such errors in positioning are known as xe2x80x9cdrop placement errorsxe2x80x9d. Although any directional inaccuracy associated with a nozzle will cause a reduction of image quality, ink jet printers are particularly sensitive to a directional inaccuracy with a direction component perpendicular to the carriage scan direction (indicated by arrow 260 in FIG. 2). This is because a nozzle that suffers from such a defect will print a row of dots which is displaced from its intended location in each swath printed by the print head. This may give rise to repeating xe2x80x9clinesxe2x80x9d on the media which have not received adequate, or possibly any ink coverage. Alternatively, it may give rise to or a line of dots of one colour incorrectly overlying an area filled by a contrasting colour. Consequently, this type of printing defect is often particularly noticeable to the human eye.
In practice this means that this type of prior art drop detection device may indicate that a given nozzle is functioning correctly, when in fact the nozzle is printing ink drops with noticeable and undesirable drop placement errors, which reduce the quality of an image. Thus, the nozzle will be used in a printing operation, without being subject to a maintenance procedure to correct the error, or alternatively not used.
A known method of determining the directionality and correct functioning of nozzles of an ink jet print head includes implementing print routines where a print head is controlled to print test patterns using known nozzles to print drops in pre-determined positions on a piece of print media. The resulting test pattern is then scanned using a line scanner built into the printer. In this manner, the scanned measurements of actual dot placements may be compared with the intended positions; thus providing information on the correct functioning, including directionality, of each nozzle. However, there are disadvantages associated with such an approach. Firstly, such tests require the use of print media, which represents an additional cost to the user of the printer device. Secondly, the printing and scanning process is comparatively time consuming. Furthermore, it is not generally possible to implement such test procedures in an automatic manner, as and when required, under the control of the printer device; i.e. without the need for operator intervention.
It would therefore be desirable to provide a system and method for correctly determining the usability of nozzles in a print head which overcomes one or more of the disadvantages associated with the prior art methods
According to the present invention there is provided an ink jet apparatus comprising a nozzle arranged to eject ink droplets and an edge detector arranged to detect droplets having a first range of trajectories and arranged not to detect droplets having a second range of trajectories, the nozzle being arranged to eject one or more first droplets from each of a plurality of positions known relative to the edge detector, the positions being arranged such that the number of first droplets detected by the edge detector varies in dependence upon the magnitude of a component of the ejection direction of the nozzle, the apparatus being arranged to substantially determine a component of the ejection direction of the nozzle in dependence upon the detection by the edge detector.
By arranging a nozzle of an ink jet apparatus to eject a series of ink drops from known positions relative to an edge or drop detector and detecting which of those drops pass through a known range of positions, as defined by the detection zone of the drop detector, it is possible to determine a direction component of the flight path of the drops relative to the nozzle; i.e. a component of the direction of ejection of the drops. Preferably, this is achieved by ejecting a series of drops in substantially the same direction, that are also ejected from substantially equally spaced positions along a line that traverses the edge of the edge detector. In this manner, a proportion only of the drops will be detected, and a component of the ejection direction of the nozzle may be determined from the detected proportion.
Preferably the apparatus is arranged to yield a two different component of the ejection direction of the nozzle in question. In this manner, components of direction of the ejected ink drops may be obtained in two orthogonal axes; for example the media feed axis and the scan axis of the printer. Preferably this is achieved by arranging two drop detectors under the scan axis of the printer, arranged at differing angles to the scan axis. Preferably, the drop detectors are arranged at 90 degrees to each other. As a printhead of the printer, comprising the nozzle in question, traverses the scan axis of the printer, a component of the direction of ejection of the nozzle may be obtained using the detection output of each the two drop detectors.
Preferably, different nozzles of the print head will are arranged to pass over each detector at different times as the print head moves in the direction of the scan axis. This means that with each pass of the printhead over a detector more than one nozzle may be tested. Thus, a large proportion, if not all, of the nozzles in a given printhead may be rapidly tested in a reduced number of passes over the drop detectors.
Preferably, the printer is arranged to pass over both the print medium and at least one of the two drop detectors in each pass along the scan axis while printing. In this manner, it is possible to test the directionality and functioning of selected nozzles of a selected printhead during the printing of an image. This allows the printer to modify the usage of tested nozzles during a print operation in dependence upon the test results for those nozzles. For example if a nozzle is found not to be ejecting ink drops or ejecting ink drops in an incorrect direction, that nozzle could be withdrawn from use for the remainder of the printing operation by allocating its work load to further nozzles. In this manner, output print quality may be increased.
Thus, the method and apparatus of the present invention may be implemented in an automatic manner, requiring no operator input. Furthermore, the directionality of nozzles of a printer may be tested without the need for the requirement for scanning print patterns printed on print media.
The present invention also extends to the corresponding method. Furthermore, the present invention also extends to a computer program arranged to implement the present invention in conjunction with suitable hardware.